A SCA7 CAG/CTG repeat expansion is stable in Drosophila melanogaster despite modulation of genomic context and gene dosage Stephen M. Jackson a , Alex J. Whitworth a , Jessica C. Greene a , Randell T. Libby b , Sandy L. Baccam b , Leo J. Pallanck a , Albert R. La Spada b,c,d, * a Department of Genome Sciences, University of Washington, Seattle, WA 98195, United States b Department of Laboratory Medicine, University of Washington, Seattle, WA 98195, United States c Department of Medicine, University of Washington, Seattle, WA 98195, United States d Department of Neurology, University of Washington, Seattle, WA 98195, United States Received 13 September 2004; received in revised form 13 November 2004; accepted 6 December 2004 Received by S.M. Mirkin Abstract CAG and CTG repeat expansions are the cause of at least a dozen inherited neurological disorders. In these so-called bdynamic mutationQ diseases, the expanded repeats display dramatic genetic instability, changing in size when transmitted through the germline and within somatic tissues. As the molecular basis of the repeat instability process remains poorly understood, modeling of repeat instability in model organisms has provided some insights into potentially involved factors, implicating especially replication and repair pathways. Studies in mice have also shown that the genomic context of the repeat sequence is required for CAG/CTG repeat instability in the case of spinocerebellar ataxia type 7 (SCA7), one of the most unstable of all CAG/CTG repeat disease loci. While most studies of repeat instability have taken a candidate gene approach, unbiased screens for factors involved in trinucleotide repeat instability have been lacking. We therefore attempted to use Drosophila melanogaster to model expanded CAG repeat instability by creating transgenic flies carrying trinucleotide repeat expansions, deriving flies with SCA7 CAG90 repeats in cDNA and genomic context. We found that SCA7 CAG90 repeats are stable in Drosophila , regardless of context. To screen for genes whose reduced function might destabilize expanded CAG repeat tracts in Drosophila , we crossed the SCA7 CAG90 repeat flies with various deficiency stocks, including lines lacking genes encoding the orthologues of flap endonuclease-1, PCNA, and MutS. In all cases, perfect repeat stability was preserved, suggesting that Drosophila may not be a suitable system for determining the molecular basis of SCA7 CAG repeat instability. D 2004 Elsevier B.V. All rights reserved. Keywords: Trinucleotide; Repeat instability; Spinocerebellar ataxia type 7; Genomic context; Trans-acting factor; Drosophila melanogaster ; Deficiency stock; Modifier screen 1. Introduction The CAG/polyglutamine repeat diseases are a family of nine devastating neurodegenerative syndromes, including spinobulbar muscular atrophy (SBMA), Huntington’s dis- ease (HD), dentatorubral-pallidoluysian atrophy (DRPLA), and six spinocerebellar ataxias (SCA1, 2, 3, 6, 7, and 17; Zoghbi and Orr, 2000; Nakamura et al., 2001). These diseases result from mutations that change tandem CAG repeats, typically 10–30 units long, into expanded CAG tracts of as few as 35 repeats to as many as 300 repeats or more. As the CAG repeats are found within the coding sequences of genes, they produce abnormally long stretches of polyglutamine which disrupt the normal function of the surrounding gene and/or produce a toxic gene product, resulting in the neurodegenerative pathology (La Spada and 0378-1119/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gene.2004.12.008 * Corresponding author. Department of Laboratory Medicine, Univer- sity of Washington, Seattle, WA 98195, United States. Tel.: +1 206 598 2138; fax: +1 206 598 6189. E-mail address: laspada@u.washington.edu (A.R. La Spada). Gene 347 (2005) 35 – 41 www.elsevier.com/locate/gene